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and vice versa. The result signified that this behavior was not a kinetic effect but
was rather due to a trapping of the system to a stationary state of local equilibrium,
with the additive not crossing to the other side of the interface. Such an explanation
was also discussed by Hu et al. for the PS-
b
-PDMS case [
48
]. It was suggested that,
in the case of graft copolymers, it is the asymmetric architecture of the graft
copolymers that leads to the great disparity between the two cases, whereas it
should probably be the asymmetry in the statistical segment lengths of the two
blocks in the PS-
b
-PDMS case that leads to an asymmetry in the CMC and, thus, in
the interfacial tension reduction.
Wedge and Wolf [
54
] discussed similar “stationary states” to be due to larger
thermodynamic driving forces and more pronounced back-damming when the
PEO-
b
-PPO-
b
-PEO triblock was added to the PPO phase. This was attributed to a
lower affinity of the additive to the PPO. Actually, the authors generalized their
finding by suggesting that, in order to achieve the highest possible reduction of the
interfacial tension by means of a given amount of compatibilizer, it should be added
to the phase with the lower affinity to this component. The study of Retsos et al. [
56
]
agreed with the statement that the effectiveness of the interfacial modifiers is
controlled by the unfavorable interactions, which drive more of the additive
towards the interface and thus reduce the interfacial tension further. However, the
study pointed to the important effect of the formation of micelles within the bulk
phase to which the compatibilizer is added, which is specifically important for
nonsymmetric copolymer architectures. One should aim at adding the compatibili-
zer to the phase where it would form micelles with greater difficulty [
56
].
As was pointed out in the article of Retsos et al. [
55
], it should be noted that the
concentration dependence of the surface tension in solvent/additive systems has
been traditionally used for the estimation of the CMC in either small-molecule
[
270
]orpolymeric[
265
,
269
,
271
,
272
] surfactant solutions. In those measure-
ments [
265
,
269
,
271
,
272
], the surface tension decreases with increasing concen-
tration for concentrations up to a certain value, and then attains an almost constant
value. The break in the
g
surf
versus log
c
(
c
is the additive concentration) curve is
used to denote the CMC. In the studies discussed above, however, it was found
that even for concentrations in the plateau region (higher than the break) of the
interfacial tension (or surface tension [
55
]) versus concentration curve, micelles
are not present for low additive molecular weights, whereas they are present only
for higher molecular weights (or equivalently for the graft copolymer case [
56
]).
Therefore, it is apparent that the break in the interfacial tension versus concentra-
tion curve should denote interfacial saturation and not necessarily micellization.
This statement is supported by an early study of solutions of PS-
b
-poly(hexyl
methyl siloxane)-
b
-PS triblock copolymer in benzene [
273
], where, although the
surface tension data exhibited the break discussed above, no micellization was
established by static light scattering. No aggregation was expected since benzene
is a good solvent for both blocks. The situation when both surface segregation
(adsorption at a solid surface) and micellization might occur was investigated
theoretically [
274
]. It was found that, depending on the incompatibility of the
surface active block with the (monomeric or polymeric) solvent and its
